This invention pertains to methods for removal of pollution from the exhaust gas stream of combustion sources. More particularly, the invention relates to an improved method of elimination of sulfur oxides and volatile toxic materials from the flue gas of coal fired boilers, especially in large electric generating plants.
Because of increasing public concern over deterioration of the environment, industries are being required by current and anticipated regulations to limit the emission of toxic materials (e.g. volatile metals or organic materials), and precursors to acid rain (sulfur oxides and nitrogen oxides) into the atmosphere. Unfortunately, many processes for controlling these pollutants are either unproven or expensive. Known sulfur oxides (SO.sub.2 and SO.sub.3) removal and collecting processes include wet scrubbers, dry scrubbers and dry powder injection. Modern particulate collection means include fabric filters (baghouses) and electrostatic precipitators. Methods of collecting volatile toxic materials are not well established, but are generally based on scrubber or fabric filter technology.
An electrostatic precipitator (ESP) consists of a series of pairs of electrodes maintained at a high voltage difference within a gas passage for ionizing the air. As dust or ash laden air passes between the electrodes, the particles are also ionized and move in the electrical fields to be collected on one of the electrodes. The material that is collected is periodically dumped into hoppers by mechanical means and removed to a disposal site. ESPs have been selected as the particulate removal means in the past due to their low cost. However, with more stringent removal requirements, alternate technologies have become competitive, since ESPs are extremely sensitive to dust properties (i.e., coal types).
Baghouses comprise an array of tubular fabric filters within a large filter casing through which flue gas containing dust or ash is drawn by a large fan. The fabric filters are cleaned periodically by pulse jet, reverse-gas flow or shaking. Pulse jet cleaned fabric filters can have a higher air to cloth ratio (3:1) than reverse-gas cleaned filters (2:1), therefore providing a size advantage over other fabric filter designs. The particles are collected in hoppers for disposal. Baghouses are relatively insensitive to the properties of the particulates and have a very high collection efficiency.
Dry scrubbing is a developing form of flue gas desulfurization (FGD) technology which utilizes a slurry, comprising sorbent material and water, to adsorb sulfur oxides and permit their removal from flue gas. Furthermore, in dry scrubbing processes, sorbent material is mixed with flue gas by being injected into plant flue gases between the combustion source and the particulate collection device. The sorbent is usually calcium based, such as calcium hydroxide (slaked lime), or hydrated dolomitic lime (CaMg(OH).sub.4 or CaMgO(OH).sub.2), although sodium-based sorbents (Nahcolite, or Trona) have also been used. Dry scrubbing systems inject calcium hydroxide or other sorbent into the duct as a slurry containing sorbent and water. In such cases evaporation of the water in the slurry cools and humidifies the flue gas contained in the duct. A dry scrubber uses a slurry of sorbent with only enough water to approach temperatures within, typically, 20.degree. F. above the adiabatic saturation temperature. Another characteristic of dry scrubbing is that the heat of the flue gas at the point of slurry injection is sufficient to evaporate all or nearly all of the water in the slurry to form a particulate of the sorbent and adsorbed sulfur oxides, and the products of the process are removed from the system as a dry powder in contrast to a liquid slurry, which is the form of the product recovered from a wet scrubber.
An example of a dry powder injection system is the HyPas test system, disclosed in U.S. Pat. No. 4,956,162, in which water is injected into the flue gas before injection of the dry sorbent powder (calcium hydroxide). There are two disadvantages to this type of process: (1) limited sorbent utilization and sulfur oxides removal occurs under most conditions, probably because a sulfite shell immediately begins to form around the solid calcium hydroxide crystals, thus imposing a diffusion barrier to sulfur oxides; and (2) corrosive gases (HCl and H.sub.2 SO.sub.4) are formed upon injection of water into the duct prior to the addition of sorbents which adsorb the corrosive gases. In general, dry powder injection systems are not as effective as dry scrubbing (slurry injection) processes at adsorbing either sulfur oxides or sulfuric acid that will be present in flue gas produced by combustion of high sulfur coals. The comparative ineffectiveness of dry powder injection is exemplified by the HyPas process shown in Table 1.
The most prevalent dry scrubbing slurry injection system is the rotary atomizer spray dryer, which injects a slurry comprising calcium hydroxide and water (25-40% solids). The atomizer produces a spray of small droplets (50-100 .mu.m) that contact the flue gas in a spray drying vessel with cyclonic gas flow. Evaporation of the water in the slurry droplets cools and humidifies the flue gas and leaves hydrated lime particles to react with the sulfur oxides in the flue gas. Typical residence time in spray drying vessels is 10 to 12 seconds. Spray dryers have been demonstrated to remove greater than 90% of the sulfur dioxide (SO.sub.2) from the flue gases from the combustion of high-sulfur coal. Dry fly ash, calcium sulfite and sulfate, and unreacted calcium hydroxide are passed from the spray drying vessel through a duct into a particulate control device.
Duct injection, in contrast to spray drying, involves direct injection of sorbent into the duct without the large reaction vessel used in spray dryers. Duct injection, as implemented in the prior art, typically removes 20% to 60% of the sulfur oxides from coal combustion flue gases. Low rates of sorbent utilization are therefore typical of existing duct injection systems. The relatively low sulfur oxides removal efficiency and low sorbent utilization efficiency of existing duct injection systems result from both the lack of residence time for adsorption reactions in a typical duct and the inability to operate at temperatures close to adiabatic saturation temperatures because of wall deposition problems.
There are many pilot and full-scale flue gas desulfurization (FGD) systems producing dry reaction products. The results of tests of some full-scale dry FGD systems are listed in Table 1. Some are dry scrubbers while others like HyPas use dry powder injection. Some of these systems use ESPs to remove sulfur oxides, while others use fabric filters (baghouses). It is noteworthy that fabric filters in these systems remove as much as 89% of the SO.sub.2 entering them. Data from the EPRI High Sulfur Test Center not included in Table 1 indicate that even higher removals of SO.sub.2 can be achieved across a fabric filter. Two cases that provide good comparisons between the contributions of fabric filters and ESPs to SO.sub.2 removal efficiency are included in Table 1. The Laramie River and Craig plants have similar Babcock & Wilcox (B&W) dry scrubbers of the spray dryer type. The ESP is credited with no removal of SO.sub.2 at Laramie River, whereas the fabric filter at Craig, even though operated at a temperature 50.degree. F. above the adiabatic saturation temperature, removes 30% of the SO.sub.2 entering it. As a second example, data from tests at TVA's Shawnee plant directly compare the SO.sub.2 removals across an ESP and a fabric filter in a high-sulfur coal application of spray dryer technology. As shown in Table 1, the fabric filter SO.sub.2 removal at Shawnee is 36% to 46x% higher than the removal in the ESP. The relative effectiveness of fabric filters in overall SO.sub.2 removal is a major reason fabric filters are the predominant particulate control device on dry FGD systems.
Of the full-scale dry scrubbing FGD systems in service, the B&W dry scrubber is the system that most resembles the duct injection process. The B&W scrubber consists of a spray drying chamber into which an array of dual-fluid nozzles (B&W I-Jet design) spray a slurry comprising calcium hydroxide and water. This system, which is installed at Craig Station in Colorado, has a 9-second residence time in a spray drying chamber. With a temperature of 18.degree. F. above the adiabatic saturation temperature at the exit of the spray drying chamber, and a calcium to sulfur ratio of 1.4 to 1, the scrubber removes about 78% of the SO.sub.2 from the flue gas. The gas is then reheated to 50.degree. F. above the adiabatic saturation temperature at the inlet to the baghouse. The fabric filter removes 30% of the SO.sub.2 entering it, boosting the system SO.sub.2 removal efficiency to 85%.
Currently, dry FGD systems located upstream of fabric filters are operated to avoid any condensation in the fabric filter. A reheating step is sometimes used to allow the scrubber to operate at lower flue gas temperatures for increased SO.sub.2 removal efficiency while protecting the downstream fabric filter from the corrosion and irreversible clogging associated with temperatures that approach the adiabatic saturation temperature of the flue gas. These potential problems have played a role in the design of current FGD and fabric filter systems. Corrosion problems have been reported in numerous installations. However, there is evidence of fabric filter temperature excursions below the adiabatic saturation temperature that did not clog or otherwise irreversibly damage the filter bags. In general, the dustcake, comprising fly ash, reacted sorbent and unreacted sorbent, formed in fabric filters downstream of dry FGD systems, is easier to clean from the filters than a dustcake of fly ash alone.
Data in Table 1 suggest that dry scrubbing FGD systems can achieve high sulfur oxides removal efficiency, and illustrate the dependence of sulfur oxides removal efficiency in dry scrubbing systems on the stoichiometric ratio of calcium to sulfur and flue gas relative humidity. Optimizing the utilization of sorbent in dry scrubbing systems requires operating at temperatures close to the adiabatic saturation temperature of the flue gas. This requirement is especially critical in duct-injection processes, where the residence time for reaction of the sorbent and sulfur oxides is necessarily shorter than the 10 to 12 seconds available in typical dry scrubbers that use large reaction vessels.
The poor utilization of sorbent in duct-injection systems operated at conventional temperatures of greater than 20.degree. F. above the saturation temperature, makes the levelized costs of these systems relatively high in spite of their substantial advantage in capital cost. Duct injection has been characterized as the process with the least capital cost of any FGD system, but among the highest operating costs in terms of dollars per ton of SO.sub.2 removed from the flue gas. For this reason, duct injection has been aimed primarily at installations that require the retrofit of an FGD system into a very constrained physical space, or that have limited life expectancy which dictates a minimum capital expense for any upgrade.
As a result of the capital cost advantages and poor sorbent utilization and removal efficiencies (high operating costs) which are associated with conventional duct injection technology, there exists a need for a dry scrubbing process that can increase the sorbent utilization and removal efficiencies.